A Zone is portion of the output page designed for easy dynamic updating via Ajax or other client-side effects. A
Zone renders out as a <div> element (or whatever is specified in the template) and may have content initially,
or may only get its content as a result of client side activity.

Often, Zones are initially invisible, in which case the visible parameter may be set to false (it defaults to true).

When a user clicks an ActionLink whose zone parameter is set, the
corresponding client-side Tapestry.ZoneManager object is located. It will update the content of the Zone's
<div> and then invoke either a show method (if the div is not visible) or an update method (if the div is
visible). The show and update parameters are the names of functions attached to the Tapestry.ElementEffect
object. Likewise, a Form component may also trigger an update of a
client-side Zone.

The server side event handler can return a Block or a component to render as the new
content on the client side. Often, re-rendering the Zone's body is useful. Multiple
client-side zones may be updated by returning a MultiZoneUpdate.

You will often want to specify the id parameter of the Zone, in addition to it's Tapestry component id; this "locks
down" the client-side id, so the same value is used even in later partial renders of the page (essential if the Zone
is nested inside another Zone). When you specify the client-side id, it is used exactly as provided (meaning that you
are responsible for ensuring that there will not be an id conflict even in the face of multiple partial renders of
the page). Failure to provide an explicit id results in a new, and non-predictable, id being generated for each
partial render, which will often result in client-side failures to locate the element to update when the Zone is
triggered.

In some cases, you may want to know (on the server side) the client id of the zone that was updated; this is passed
as part of the Ajax request, as the QueryParameterConstants.ZONE_ID parameter. An example use of this would
be to provide new content into a Zone that updates the same Zone, when the Zone's client-side id is dynamically
allocated (rather than statically defined). In most cases, however, the programmer is responsible for assigning a
specific client-side id, via the id parameter.

A Zone starts and stops a Heartbeat when it renders (both normally, and when re-rendering).

After the client-side content is updated, a client-side event is fired on the zone's element. The constant
Tapestry.ZONE_UPDATED_EVENT can be used to listen to the event.

The element name to render for the zone; this defaults to the element actually used in the template, or "div" if
no specific element was specified.

String

Required, Not Null

literal

id

If bound, then the id attribute of the rendered element will be this exact value. If not bound, then a unique id
is generated for the element.

String

literal

show

Name of a function on the client-side Tapestry.ElementEffect object that is invoked to make the Zone's
div visible before being updated. If not specified, then the basic "show" method is used.

String

symbol:tapestry.components.zone_show_method

literal

update

Name of a function on the client-side Tapestry.ElementEffect object that is invoked after the Zone's content has
been updated. If not specified, then the basic "highlight" method is used, which performs a classic "yellow fade"
to indicate to the user that and update has taken place.

String

symbol:tapestry.components.zone_update_method

literal

visible

If true (the default) then the zone will render normally. If false, then the "t-invisible" CSS class is added,
which will make the zone initially invisible.

It's pretty standard to bind the id parameter of a zone; the zone parameter of ActionLink
and Form are theclient-side element id, not the component id. They are often,
but not always, the same. Binding the id parameter ensures that a particular, fixed value
is used.

Binding the zone parameter of the ActionLink, EventLink or Form (the "trigger"
component) is what triggers the partial-render and update logic.

ZoneDemo.java

The event handler method for the clicker component increments the count, then
returns the body of the zone. The body will be re-rendered (reflecting the new count),
and sent to the client, which will update the zone in place, and trigger and animation
(by default, a simple yellow fade) to draw the user's attention.

Not shown here, but fully valid, is to include JavaScript libraries and generate
initialization JavaScript. This fully consistent with ordinary full-page renders.

Zone

getClientId

The client id of the Zone; this is set when the Zone renders and will either be the value bound to the id
parameter, or an allocated unique id. When the id parameter is bound, this value is always accurate.
When the id parameter is not bound, the clientId is set during the begin render phase
and will be null or inaccurate before then.

getBody

Returns the zone's body (the content enclosed by its start and end tags). This is often used as part of an Ajax
partial page render to update the client with a fresh render of the content inside the zone.